Hello Roland,
 
Thanks for the information.  As I read your post, a few questions
came to mind, some more serious than others:
How do you define an "occupied bottle" - does it actually have to
have an octopus in it when you find it to qualify as occupied?  I'm
kind of confused by the terminology here, because you speak of
relative numbers of shells in occupied vs. unoccupied bottles.
Presumably the presence of any shells indicates that the bottle was
occupied at some point?
If the octopus favor brown and encrusted bottles, presumably darkness
is the condition they seek?  Is this therefore a shallow  water
species?
Where did this species live before beer bottles arrived on the scene?
 Do they still live there?  Is the species found in areas that are not
littered with bottles?  Are there enough "naturally occurring" (so to
speak) beer bottles to support the population, or did you have to
"seed" an area for the sake of the study?
In your 6-month experiment, will your final examination for
deterioration be based on weight loss, or just visual evidence?
Could the dissolution of shells in occupied bottles be due to, or
accelerated by, acids resulting from:
(1) waste products of the octopus (anybody know the pH of octopus
poop?)
(2) carbonic acid formed from CO2 produced by the respiration of the
octopus in such a small, restricted space?
(3) decomposition of snail tissues not thoroughly removed from the
shells by the octopus?
Just a few thoughts.  Don't feel obligated to answer - I'm not trying
to make work for you.
 
Regards,
Paul M.